This will be an interesting topic.

Rui is the eldest of the Kisugi sisters, a trio of thieves making up Cat's Eye targeting artworks belonging to their missing father. While Hitomi is the actual lead, the narrative being her relationship with Toshi the Detective who is tasked with catching Cat's Eye, while Ai is the youngest and techy but is still in school.
Rui's role is obtaining the information and planning the heists making her the leader.
They use a hidden in plain sight disguise as baristas in a coffee shop.
Now onto the sexy part of Rui's description.
Sexy is not a word I would use to describe any character in anime, it's not really as glamourous as other pop culture media, yes, you can look sexy and act sexy but quite often it doesn't complete the full package of the character, a simple ara ara and an hourglass figure isn't enough and there's also the subjective element to it as well.
Rui looks sexy, acts sexy and feels sexy, even the picture gives off alluring bedroom eyes. But the most important element for a sexy character, they are near impossible to criticise, less than five minutes of Rui and you are already engrossed in what she's doing.
So who else would I describe as sexy?
You might be surprised but there is one anime with tons of sexy characters, that'll be a top ten and tomorrow's post.